Primarily in the field of manufacturing integrated circuits having pattern sizes far less than one micrometer, it is necessary to position the semiconductor substrates (also referred to as wafers or workpieces) very precisely under a tool such as the lens of a light-exposing apparatus, for example, with whose aid the finest patterns are then transferred into a photosensitive resist applied beforehand on the wafer.
To that end, it is described in U.S. Pat. No. 7,483,120, for example, to place the wafer on a movable table that is able to be positioned relative to the lens of the light-exposing apparatus. Four grating plates are provided as measuring standards about the lens in an X-Y plane, and are joined as rigidly as possible to the lens. The optical axis of the lens is perpendicular to the plane of the grating plates and defines a Z-direction. The table and wafer are arranged parallel to the grating plates or measuring standards. Disposed in the corners of the table are scanning heads of position-measuring devices, which scan the grating plates with the aid of light. When the table moves relative to the grating plates, the scanning heads form periodic signals from which, by counting the periods and by fine subdivision of individual periods (interpolation), extremely precise values are obtained for the change in position. If an absolute position is ascertained one time, e.g., by detecting reference marks, the determination of the change in position is synonymous with the determination of an absolute position, since the absolute position may be calculated from the change in position, starting from the reference position.
In the present context, “corners of the table” refers to edge regions of the table different from each other and set apart as far as possible from each other for a given table size. The placement of scanning heads in such corners is expedient for various reasons. Scanning heads can only be placed outside of the area occupied by the workpiece (disposed centrally on the table). In addition, as great a spacing of the scanning heads as possible among one another permits a more precise calculation of rotations from the linear shifts measured in the corners. Furthermore, individual corners or edge regions of the table may travel into the area of the tool, so that a scanning head located there is no longer able to scan a grating plate. In order to constantly determine position with extreme accuracy, it is wise to ensure that at all times, scanning heads are able to scan from areas of the table as far away from each other as possible. For this, it may be advantageous to combine the scanning heads in several corner areas, from which never more than one is able to move into the area of the tool.
Scanning heads for various measuring directions and arranged close together in one such corner reduce the size needed for the scanning plates. Ideally, the measurements may also be integrated in a single scanning head, for which examples are described below.
In the present context, two scanning heads for different measuring directions are located in the same corner of the table when their spacing is small in comparison to the expanse of the table. If two scanning heads have a spacing which is comparable to the dimensions of the table, then they are located in different corners or edge regions. At any rate, two scanning heads are located in different corners of the table when their spacing is greater than one tenth of the expanse of the table. For round tables, their diameter may be considered as an expanse, for rectangular or square tables, their diagonal may be considered as an expanse.
To position the table in the X-Y plane, its degrees of freedom must be determined in this plane. They are the linear shifts in the X-direction and Y-direction, as well as rotation rZ about the Z-axis, which together are also referred to as in-plane degrees of freedom, because all three degrees of freedom are located in the X-Y plane. To determine these three degrees of freedom X, Y, rZ, it is sufficient, for example, to measure the shift in X in two corners of the table, and in a further corner, the shift in Y. Rotation rZ may then easily be calculated. However, since during the shift of the table in the course of the exposure of the wafer, an individual corner may move into regions close to the lens in which no scanning plate is able to be scanned any longer, each of the corners of the table should have a scanning head. In U.S. Pat. No. 7,483,120, it is also described that it is advantageous in at least one of the corners to measure not only the shift in the X-direction, but also the shift in the Y-direction. Redundant measurements may be utilized to increase the measuring accuracy by averaging, or perhaps, for example, to take thermal expansions or vibrations of the table into account.
U.S. Pat. No. 7,483,120 further mentions that in the corners, in each case the distance to the grating plates may also be measured, e.g., a position measurement of the table corners in the Z-direction. With these measurements, the remaining three degrees of freedom of the table may also be determined, e.g., the linear shift in the Z-direction and rotations rX and rY about the X-axis and Y-axis, respectively. Thus, in addition, at all times, measurements in all three spatial directions are available in one corner area as close as possible to the place of action of the tool (the Tool Center Point). Due to such a 3-D position detection in one corner, accuracy of the positioning of the processing tool relative to the workpiece increases.
An optical position-measuring device suitable for such practical applications is described in European Published Patent Application No. 1 762 828. It includes a measuring standard (one of the grating plates), as well as a scanning head for scanning the measuring standard, the scanning head being situated in the corner of the table.
The scanning head permits simultaneous determination of position along a lateral shift direction (for example, an X-direction) and along a vertical shift direction (Z) of the table. The scanning head thus has two measuring axes. In order to determine position in the lateral and vertical shift direction, a first and a second scanning beam path are formed (one scanning beam path for each measuring axis), in which in each case from two non-mirror-symmetric, interfering partial beams of rays, a group of phase-shifted signals is able to be generated on the output side, which interfere with each other and produce periodic signals in a photodetector.
In highly precise position-measuring devices of the type discussed above, it is necessary to subject the periodic signals generated in the scanning head to a compensation with regard to their amplitude, their offset, and their phase relation, since only then is a very fine determination of position possible within one signal period. Since this compensation proceeds continuously during measuring operation, it is also called online compensation.
In the position-measuring device described in European Published Patent Application No. 1 762 828, care is therefore taken that even in the case of a pure shift in the lateral shift direction, periodic signals are formed in all photodetectors, even though there is no movement in the vertical shift direction.
In European Published Patent Application No. 1 762 828, this is achieved in that the two sensitivity vectors of the measuring axes of the scanning head do not point exactly in the lateral and vertical direction, respectively, but rather are disposed at specific angles relative to these directions. The sensitivity vector of a measuring axis indicates the moving direction in which the position signal of the respective measuring axis increases the fastest per unit of length traveled. Thus, it describes a property of the measuring axis. By offsetting the two periodic signals acquired, the actual movement in the desired measuring directions (lateral and vertical) may be obtained.